1. Field of the Invention
This invention relates to an internal mixer which is suitable for mixing various materials such as rubber, plastics, ceramics and the like.
2. Description of the Prior Art
Shown in FIG. 5 is a typical example of the internal mixers of this sort which have thus far been known and used in the art, including a couple of mixing rotors 2 which are connected to a drive source, not shown, and rotatably accommodated in a mixing chamber 1 in a fluid tight condition. A floating weight 4 is vertically movably fitted in a hopper frame 3 which is extended upward from the mixing chamber 1. The floating weight 4 is suspended on the distal end of a piston rod 6 which is extended downward from an air cylinder 5 to move the floating weight 4 up and down with pneumatic pressure. When the floating weight 4 is in the upper lifted position as shown in FIG. 5 (in a non-mixing stage), the mixing chamber 1 and charge hopper 7 are in an open state to receive a charge of mixing material in the mixing chamber 1. On the other hand, when the floating weight 4 is in the lower position (in a mixing stage), the mixing chamber 1 is closed, secluded from the hopper 7, while the floating weight 4 is pushed against the charged material in the mixing chamber 1 by pneumatic pressure and caused to float up and down repeatedly according to the movements of the charged material which is being mixed by the rotors 2. The upward and downward floating or sinking actions of the floating weight 4 accelerate the mixing of the charged material favorably to produce sufficient mixing and dispersing effects.
However, in a case where the above-mentioned prior art mixer is used, for example, in a vehicular tire manufacturing plant under such an operating condition where the floating weight 4 is moved up and down at a frequency of more than one time per minute, the operation involves consumption of a large quantity of compressed air which is extremely uneconomical. Besides, in a case where a high pressure is applied to the mixing material through the floating weight 4 in an initial stage of the mixing operation for the purpose of producing accelerated mixing effects to attain higher productivity, difficulties are encountered in carrying out the operation in an efficient and economical manner because it takes time to build up a high pressure due to the influence of the compressibility of air in the air cylinder of a large capacity despite the increased consumption of compressed air.
There has been another problem that, when lifting up and down the floating weight 4 or when pressing the mixing material with the floating weight 4, powder of the mixing material erupts into upper portions of the hopper frame 3 through the clearance around the floating weight 4 and deposits on the upper surface of the floating weight 4. Therefore, in order to ensure that the entire amount of the charged material be mixed and dispersed into a final product, the deposited powder has to be swept into the mixing chamber 1 at predetermined time intervals although it is very troublesome from the standpoint of machine operation. For instance, in the conventional batch type internal mixer, the powder which has been blown up in the mixing stage and deposited on the upper surface of the floating weight needs to be swept off by lifting up the floating weight one or two times before completion of one batch of mixing operation, prolonging the mixing time and as a result inevitably lowering the productivity by interruptions of the operation.